The present invention relates to method and apparatus for the chemical suppression of eluents used in the analysis of anions or cations in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,956,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin bed. A different form of suppressor column is described and published in EPA Pub. No. 32,770, published July 29, 1981, in which a charged membrane in the form of a fiber or sheet is used in place of the resin bed. In sheet form, the sample and eluent are passed on one side of the sheet with a flowing regenerant on the other side of the sheet. The sheet comprises an ion exchange membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
An improved membrane suppressor device is disclosed in EPA Pub. No. 75,371, published Mar. 30, 1983. There, a hollow fiber suppressor is packed with polymer beads to reduce band spreading. There is a suggestion that such packing may be used with other membrane forms. Furthermore, there is a suggestion that the function of the fiber suppressor is improved by using ionically charged packing beads. No theory is set forth as to why such charged particles would function in an improved manner.
Another suppression system is disclosed in EPA Pub. No. 69,285, published Jan. 12, 1983. There, the effluent from a chromatographic column is passed through a flow channel defined by flat membranes on both sides of the channel. On the opposite sides of both membranes are channels through which the regenerant solutions are passed. As with the fiber suppressor, the flat membranes pass ions of the same charge as the exchangeable ions of the membrane. An electric field is passed between electrodes on opposite sides of the effluent channel to increase the mobility of the ion exchange. One problem with this electrodialytic membrane suppressor system is that very high voltages (50-500 volts DC) are required. As the liquid stream becomes deionized, electrical resistance increases, resulting in substantial heat production. Such heat is detrimental to effective detection because it greatly increases noise and decreases sensitivity.
Charged fiber screens have been suggested for placement in a flow channel between oppositely charged membranes in the field of electrodialysis to improve current efficiency for desalination. (Desalination 19 (1976) 465-470) The charges on individual fibers are either cationic or anionic so that only fibers of one charge contacts the correspondingly charged permselective membrane in a stack. There is no suggestion that such screen would have any applicability to an analytical system.